Light pens have achieved wide acceptance as data input devices for computers in many application fields. Light pens operate by detecting the scan spot on a raster scanned cathode my tube display, as it is held in place by the user at a desired location on the display and its switch is activated by the user. From this location information, system and application software can make use of the operator's light pen input for numerous tasks such as menu or mode selection, and some graphic manipulations. The direct relationship between a light pen and a CRT image provides advantages in user effectiveness over more indirect pointing devices such as a mouse or keyboard cursor controls. Usually the activation switch is incorporated into the tip of the light pen, so that pushing the pen against the screen activates the select switch, and the travel or resistance of the tip movement provides tactile feedback to the user.
Although prior art light pens have been successful for their intended task such as menu and mode selections and the like, they have generally not been a suitable input medium for tasks requiting a light touch and delicate control, such as handwriting or direct drawing input. The reason for this has been the characteristics of the tip switch used on light pens. Typically the throw, i.e. the length of travel, of the tip switch, in combination with the return spring force, renders the pen somewhat awkward and unnatural to use for things such as handwriting. Most users are accustomed during handwriting to the very light touch afforded by a pen or pencil on paper, and the slightly harder push to keep the light pen switch actuated interferes with the natural feel and, hence, the accuracy and user acceptability of using such devices for handwriting input, or detail drawing input. However, the increasing use of graphic interfaces, and the increasing acceptance and sophistication of draw and computer-aided design types of programs make it desirable to provide a more convenient way for a use to use a light pen for such tasks.
However, a simple attempt to decrease the tip switch travel or spring resistance leads to difficulties which must be overcome. One difficulty is that a very sensitive light pen, such as might be useful for handwriting input, can become too sensitive for other types of uses such as menu or mode selection functions. When operating in that manner, a user may inadvertently touch the tip or drag it along the CRT screen while moving between locations of intended input actuation, and with a very sensitive tip, this may cause false or unwanted triggering inputs. Since, by definition, the resistance or travel of the tip switch has been greatly reduced in order to make handwriting feel natural, there would hardly be any tactile feedback remaining for the user to feel when he or she is making an intended switch actuation-input. It would therefore be necessary to exaggerate the steps of lifting and touching the pen in order to avoid the unwanted triggering. It is believed that users would find this annoying and unacceptable. Another set of problems to be overcome in designing a light pen tip switch usable for handwriting or drawing input is that the very small travel involved makes it difficult to ensure positive and reliable switching. Mechanical switches are ruled out, and even magnetic and opto-electrical switches can have difficulty in distinguishing slight movements of only 5 to 10 thousandths of an inch, particularly in an environment in which the pen and the switch must be very rugged to withstand anticipated shocks such as will be caused by dropping the pen.